Carbon-manganese steels are usually used for railroad track applications. The strength and toughness of steels used for forming such rails are controlled by alloying additions. In particular, high strength is achieved by additions of carbon, manganese, nickel and chromium, with carbon having the biggest effect. Adding nickel will also promote toughness, while decreasing sulfur and phosphorus content of the steel will also promote toughness.
The two types of steels that are most widely used in the railroad industry are pearlitic and austenitic manganese steels. Pearlitic steel provides high strength and wear resistance. Austenitic manganese steels are usually used in railway frogs because they exhibit high toughness, resistance to wear and impact loading. In recent years, bainitic steels have also been considered as a candidate material for railway tracks due to their unique mechanical properties.
Applicants previously published studies regarding the microstructure of pearlitic rail steels that are presently being used in the railroad industry in the United States and found that these steels consist of a fine lamellar aggregate of very soft and ductile ferrite and very hard carbide, cementite (Aglan et al., J. Mat. Processing Tech., 2004, 151:268-274). These lamellae are aligned in the same orientation in one grain with each grain having an average size of 50 μm. The typical carbon content of the pearlitic steel is about 0.79% with an increase in carbon content, up to about 1%, giving a higher hardness.
Austenite manganese steel (“AMS”) is currently used for crossings and frogs in the railroad industry. Crossings and frogs are considered to be very important as they are used to change the direction of the moving train. AMS is known to be extremely tough and durable with very good wear resistance properties and can withstand high impacts without catastrophic failure. At high temperature, however, AMS can dissolve more than 1% carbon within the austenitic microstructure. If the steel is cooled rapidly, all the carbon is retained in the solution. However, if it is cooled too slowly, carbide precipitation will occur at the grain boundaries and cause severe embrittlement.
Steel having a bainitic microstructure is comprised of a metastable aggregate of ferrite and cementite which is produced from the transformation of austenite at temperatures above the martensite starting temperature and below the pearlite range. Specific alloys are added to bainitic rail steels to enhance the formation of bainite. Bainitic rail steels are alloyed with approximately 0.5% molybdenum and trace amounts of boron. Molybdenum is added to delay the formation of ferrite and pearlite, boron to affect the transformation time, and manganese, nickel and chromium to decrease the bainitic transformation temperature.
One particular bainitic steel alloy that has become popular for railroad industry used is labeled J6, and has typical carbon content of approximately 0.26%. The J6 rail steel microstructure is bainite, having an average grain size of approximately 90 μm and a hardness between about 415-430 Brinell hardness (“HB”). This hardness increases the wear resistance of rail steels, but it also means that rail formed from such bainitic steels are more vulnerable to hydrogen cracking. Therefore, care is needed to control the hydrogen content of the bainitic rail steels. The bainitic rail steel is not as tough as AMS and is not able to endure the large cracks that AMS can tolerate.
When compared to conventional AMS, bainitic steels have an increase in hardness and strength, better resistance to deformation, better wear resistance and fewer casting defects. The bainitic structure also has an advantage over the pearlitic structure from the perspective of crack initiation and crack growth sites. This is because the bainitic structures do not have strong directional anisotropy which implies there are fewer sites for cracking to exist in the material. Bainitic steels, however, are also relatively more costly.
Fatigue failure of steel rail parts is the main cause of derailments and other severe railroad accidents. Because weld repairs to rail can alter the overall strength (i.e., hardness and toughness) of a rail, welding procedures need to be developed that provide optimum weld repair strength within cost and time constraints. Because of the unique rigors to which rail heads are exposed, it is important that any weld provide similar strength and flexibility characteristics to the parent steel. Studies have shown that rail welds are generally are more prone to defects than the parent rail and hence failure, because they are usually weaker than the rails they join (see, for example, Skyttebol et al., Eng. Fracture Mech., 2004, 72:271-285; and Desimone et al., Int'l J. of Fatigue, 2006, 28:635-642). Inclusions, porosity, lack of fusion or other types of defects that may be present in the welded rail are the main crack initiation sites for weld fatigue failures. These defects may be small but are locations where fatigue cracks may initiate, propagate and lead to rail failure.
The microstructure of different steel alloys closely correlates to their mechanical properties. The toughness of carbon steels is typically dependent on the austenite grain size, where a decrease in the grain size will increase the fracture toughness. In most cases, an increase in the strength of steels decreases their toughness. Increasing the nickel content of welds is known to increase the welds' toughness and ductility. This is achieved by the reduction of ferrite, which in turn increases the austenitic content. The austenite structure is stabilized by the nickel content, which helps to prevent the formation of martensite. These conditions are favorable because the austenitic phase typically has high toughness and ductility when compared to martensite.
The hardness of a particular steel sample is also dependent on the constituent microstructures that are present, with increases in martensite and ledeburite giving an increased hardness. An increase in the cooling rate of a work piece will provide higher hardness values, but a cooling rate that is too fast may cause the base material to crack, especially in high carbon steels. The hardness of the welded pearlitic steel also tends to be lower than the corresponding parent rail steel. Martensitic and austenitic steel phases are harder than the terrific phase, which is the major phase present in the pearlitic weld without heat treatment. The thermal cycle associated with welding may cause the mechanical properties in the weld material and parent steel adjacent to the weld to be degraded by grain coarsening, precipitation and by segregation of trace impurities.
In particular, for a welded steel sample there are typically four distinct homogeneous zones. These zones include (working from the site of the weld outward) the weld zone, the coarse grain heat affected zone (“HAZ”), the fine grain HAZ and the parent material. These zones exist because when a single weld bead is laid on a metal, heat from this process can transform the microstructure of the adjacent original steel to austenite. Additionally, subsequent rapid cooling can then transform the austenite to martensite, which is not a preferred microstructure for most steel applications. If a sequence of several weld deposits, called multi-runs, is used for welding, the microstructure of the weld becomes much more complicated as the deposition of each successive layer of weld heat treats the underlying microstructure. The multi-runs may also temper the weld, which subsequently alters its mechanical properties.
Thus, heat input in particular is a very important characteristic of rail welding because it influences the heating and cooling related phase changes in the weld material and affects the microstructure and mechanical properties of the weld metal and the immediately adjacent HAZs. Heat input can be approximately characterized as the ratio of the arc power supplied to the electrode to the velocity of the heat source. Further, once welding is completed any heat treatment that the material undergoes is referred to as post-weld heat treatment (“PWHT”). Generally, this is done in welding to either improve the mechanical properties of the weld or to help in the prevention of defects, such as to increase resistance to brittle fracture, increase the strength of the material, and/or relax residual stresses present in the weldment. The use of PWHT, however, varies significantly from application to application and can effect material properties and microstructure.
Each of the various steels used to form rail parts thus introduce weldability challenges, especially in the case of in-situ repairs on railroad lines. Currently, flash-butt welding (“FBW”) and thermite welding processes are the most commonly employed, but both are very expensive and time consuming. To affect a rail head repair via flash-butt or thermite welding, upon detecting a defect in the railway track, a rail section of approximately 6 meters in length containing the defect is removed and replaced with a new rail of the same steel and then welded into place. The major drawbacks of both thermite welding and FBW, including cost, time, weakening of the rail head, and ineffective control of microstructural changes due to thermal effects, have led to a current need for a more cost effective, efficient and practically viable methodology for the in-situ repair of rail defects.
Slot repair of rail head defects has been proposed as an alternative to FBW and thermite welding, but to date this approach has not been successfully adapted to rail applications. For example, a prior study by Applicants regarding slot welding of rail heads of pearlitic steel rails found that slot welding produced a strength and hardness mismatch between the parent rail material and the weld material, with the weld being both lower in strength and hardness (see Aglan, “Fracture and Fatigue Evaluation of Slot-Welded Railhead Repairs,” Federal Railroad Administration Report RR08-26, November 2008). All welded samples tested in that study failed at the fusion line, indicating that the slot welding process used therein didn't obtain proper fusion between the weld and the parent rail steel.